Despite the significant morbidity and mortality caused by human polycystic kidney diseases, the mechanisms by which cysts form and progressively enlarge in such disease states are largely unknown. The overall objective of the application is to identify the pathophysiological processes which mediate the formation and progressive enlargement of tubular cysts in a murine model of human autosomal recessive polycystic kidney disease, the C57BL/6J cpk/cpk mouse (CPK). On the basis of preliminary in vivo and in vitro studies in this model, an overall experimental hypothesis has been generated which links abnormal expression and/or membrane targeting of two specific proteins - the Epidermal Growth Factor Receptor (EGF-R) and the NaK Adenosine Triphosphatase (NaK ADPase)-with tubular epithelial hyperplasia, altered transtubular transport, and consequent tubular cyst formation and progressive enlargement. Studies focused on four specific aims will critically evaluate the experimental hypothesis: I. To characterize EGF-R expression in proximal tubule and collecting tubule epithelial cells at progressive stages of CPK cyst formation; II. To characterize the role of EGF-R activation in cystic epithelial hyperplasia utilizing EGF and TGF-alpha as specific ligands; III. To characterize NaK ATPase expression in proximal tubule cells at progressive stages of CPK cyst formation; and IV. To characterize the mechanisms by which increased NaK ATPase activity or altered location in cystic epithelia may lead to altered solute transport. Studies will be performed in whole kidneys, cell cultures of tubular epithelia derived from discrete control or cystic nephron segments at specific developmental stages, and organ culture models of tubular cyst modulation. In these models, specific studies will focus on EGF-R and NaK ATPase subunit gene expression, protein targeting and turnover in specific plasma membrane domains, and protein expression and activity in defined epithelia at progressive stages of normal and cystic tubulogenesis. The proposed studies will identify primary pathogenic processes which initiate and promote cyst formation in a murine model of autosomal recessive polycystic kidney disease. The identification of specific cyst-initiating and cyst-promoting processes in this animal model may provide insight into the pathogenesis of renal cyst formation in human Autosomal Recessive Polycystic Kidney Disease and other human disease states.